I am currently working on an Excel 2010 add-in which formerly was an Excel 2007 add-in.
Somewhere in the process of switching computers, the add-in was converted i think.

Some of my customers stated that the add-in wasn't working on Excel 2007 anymore so I tried to debug it in a VirtualBox with Excel 2007 and Visual Studio 2010 installed.

Now I get the Error Message:

You cannot debug or run this project, because the required version of the Microsft Office application is not installed.

I started a new Excel 2007 add-in project and tried to find what the differences are and came up with the idea that it somewhat has to do with the dll's so I changed my 2010 addin until it looked like an 2007 addin.

I still get the error message stating that my project can't be debugged.

It looks like the # symbols are just enclosing a registry path -- it looks like it's basically saying "find this path from the registry and append excel.exe to it." -- So I decided to clone my registry settings from the 12.0 hive into the 14.0 hive and give it a shot. Lo' and behold! It works! -- I can now, with one .sln/.csproj file open and debug the same add-in on my Windows XP/Office 2007 VM as I can on my Windows 7/Office 2010 VM (using VSTO 4.0 of course).
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BrainSlugs83Jan 16 '13 at 23:14

Typically, when I am developing against multiple versions of Office with VSTO Add-ins, I have a project for each version of Office I am targeting. I put all common code between the projects into a single project (typically the oldest project) and use linked files, I add the common files to the newer projects. This allows me to write one set of common core code, abstracted from the requirements of each version of Office. This means I am no longer fighting the different ways VSTO is compiled for each version of Office. This can be made easier with shared folders and virtual machines, so I can develop and test without multiple computers. It is by no means graceful, but it works well for me. This should allow you to develop your VSTO Add-in against both Office 2007 and Office 2010 without much issue.